Acousto-optic modulators, sometimes referred to as Bragg cells, diffract and shift light using sound waves at radio frequency. These devices are often used for Q-switching, signal modulation in telecommunications systems, laser scanning and beam intensity control, frequency shifting, and wavelength filtering in spectroscopy systems. Many other applications lend themselves to using acousto-optic devices.
In such acousto-optic devices, a piezoelectric transducer, sometimes also referred to as an RF transducer, is secured to an acousto-optic bulk medium as a transparent optical material, for example, fused silica, quartz or similar glass material. An electric RF signal oscillates and drives the transducer to vibrate and create sound waves within the transparent medium which effect the properties of an optical field in the medium via the photo elastic effect, in which a modulating strain field of an ultrasonic wave is coupled to an index of refraction for the acousto-optic bulk medium. As a result, the refractive index change in amplitude is proportional to that of sound.
The index of refraction is changed by moving periodic planes of expansion and compression in the acousto-optic bulk material. Incoming light scatters because of the resulting periodic index modulation and interference, similar to Bragg diffraction.
A piezoelectric transducer can create the sound waves and the light beam is diffracted into several orders. It is possible to vibrate a bulk medium with a sinusoid signal and tilt the acousto-optic modulator such that light is reflected from flat sound waves into a first diffraction order to result in a high deflection efficiency.
In acousto-optic devices, light can usually be controlled by: 1) deflection, 2) intensity, 3) frequency, 4) phase, and 5) polarization.
In acousto-optic systems using deflection, a diffracted beam emerges at an angle depending on the wavelength of the light relative to the wavelength from the sound. When controlling light by intensity, however, the amount of light diffracted by sound depends on the intensity of the sound to modulate the intensity of the light in the diffracted beam. With frequency control over the light, the frequency of the diffracted beam is Doppler-shifted by an amount equal to the frequency of the sound wave, as compared to Bragg diffraction, in which light is scattered from moving planes. The frequency shift can also occur because the energy and momentum of photons and phonons is conserved. Frequency shifts can vary from as little as 20 MHz to as much as 400 MHz or with even greater ranges in some cases. Two acoustic waves can travel in opposite directions in the material and create a standing waves which does not shift the frequency. In systems controlling light using phase, the diffracted beam can be shifted by the phase of the sound wave. In systems controlling light by polarization, collinear transversal acoustic waves are perpendicular along longitudinal waves to change polarization. Thus, there could occur a birefringent phase-shift.
Acousto-optic modulators are preferred in many applications because they are faster than tiltable mirrors and other mechanical devices. The time it takes for the acousto-optic modulator to shift an exiting optical beam is limited to the transit time of the sound wave. The acousto-optic modulators are often used in Q-switches where a laser produces a pulsed output beam at high peak power, typically in the Gigawatt range. This output could be higher than lasers operating a continuous wave (CW) or constant output mode.
Examples of acousto-optic modulator devices and similar acousto-optic systems are disclosed in commonly assigned U.S. Pat. Nos. 4,256,362; 5,923,460; 6,320,989; 6,487,324; 6,538,690; 6,765,709; and 6,870,658, the disclosures which are hereby incorporated by reference in their entirety.
Conventional acousto-optic devices typically rely on the use of large and costly hybrid output radio frequency (RF) amplifiers to provide the requisite drive power necessary for use in operation of the device. Usually a higher supply voltage is applied in order to accommodate the requisite output intercept point of the hybrid output radio frequency amplifiers. This results in the use of additional and/or more costly power supplies to meet both the integrated circuit (IC) and radio frequency amplifier requirements.
In other applications of conventional acousto-optic devices, the designs typically incorporate one or more monolithic piezoelectric platelets, which are bonded to the bulk medium for launching an ultrasonic strain field into the bulk medium. In some examples, a low compliance alloy bond fuses the two components together providing an interface, which results in lower acoustic losses, while accommodating broadband impedance matching between the platelet and the optical, bulk medium. The combination of different coefficient-of-thermal expansion (CTE) for the platelet and optical medium, coupled by the low compliance interface, may lead to stress causing localized shear CTE expansion mismatch induced fracture and failure of large platelet acousto-optic devices subjected to extended temperature conditions. These extended temperature conditions may occur both in a non-operable state, i.e., at a survivability storage temperature, or in some cases as a result of high-level signal conditions.
Some critical applications using acousto-optic devices modulate the intensity of an optical beam. This modulation can create small deviations in the output angle of the diffracted beam because of the local thermal transients introduced when the RF modulation waveform to the device is turned ON and OFF. These thermal transients can negatively impact the resolution of the focused spot, which can be produced.